The present invention relates to circuits for the protection of a MOS output drive from electrical overstress (EOS) or electrostatic discharge (ES) stress, and, more particularly, to circuits for the dynamic turn off of an NMOS output driver during EOS/ESD stress.
An NMOS output driver is subject to being damaged when subjected to EOS/ESD stress. Heretofore, for EOS/ESD protection of an I/O pad, either the NMOS output drivers was designed to be self-protecting, or an electrical EOS/ESD clamp was added in parallel to the NMOS output driver. For various reasons the use of an electrical EOS/ESD clamp is preferred for this purpose. However, when an electrical EOS/ESD clamp is added, a problem can arise when the EOS or ESD stress is applied to the I/O pad. There are several stress situations where as a result of the application of a transient over voltage stress at the I/O pad, the circuit or part of the circuit gets powered up dynamically. In such situations, the pre-drive circuits will force either a high or low state at the gate of the final output drivers which is undesirable.
A circuit for the protection of an output driver NMOS transistor during EOS/ESD stress includes an output driver NMOS transistor and an output driver PMOS transistor connected in series between a Vdd line and a Vss line with the gates of the output driver transistors being connected together. An I/O pad is connected to the junction between the output driver transistors. A pre-driver stage includes an NMOS transistor and a PMOS transistor connected in series between the Vss line and the Vdd line with the gates of the pre-driver transistors being connected together. The output of the pro driver stage is connected to the gates of the output driver transistors. A gate clamp has a first connection to the Vss line, a second connection to the junction between the pre-driver transistors, a third connection to the I/O pad and a fourth connection to the gate of the output driver NMOS transistor. An ESD clamp is connected between the I/O pad and the Vss line and has a connection to the third connection of the gate clamp.